You cannot force Google to use any particular bit of text as their blurb on the SERPs. Google chooses what it wants to display. Sometimes it's your meta description, sometimes it's a piece of text from the page.

I thought that's what AIOSEO was supposed to do.

No plug-in can do that. Google's algorithms alone decide what will be displayed. AIOSEO simply lets you add a custom meta description per page, which Google may or may not use, at its sole discretion.